ABINIT Input Files for Hydrogen Simulations

Introduction to ABINIT Inputs

The provided content is a plain text file that serves as the main input for an ABINIT calculation. It is organised into several datasets, separated by ---. Each dataset instructs ABINIT to perform a specific task. In this example, we will examine calculations for a Hydrogen atom or molecule.

Dataset 1: Basic Ground-State Calculation

The first dataset defines the fundamental parameters for the simulation.

acell 10 10 10
ntypat 1
znucl 1
pp_dirpath "$ABI_PSPDIR"
pseudos "Psdj_nc_sr_04_pw_std_psp8/H.psp8"

natom 2
typat 1 1
xcart
  -0.7 0.0 0.0
   0.7 0.0 0.0

ecut 10.0
kptopt 0
nkpt 1

nstep 10
toldfe 1.0d-6
diemac 2.0

• acell 10 10 10: Defines the size of the simulation box (the unit cell) in Bohrs.
• ntypat 1 / znucl 1: Specifies one type of atom with an atomic number (charge of the nucleus) of 1, which is Hydrogen.
• pseudos: Points to the pseudopotential file for Hydrogen, which simplifies the calculation by replacing the core electron with an effective potential.
• natom 2 / xcart: We are simulating two atoms, and their initial positions are given in Cartesian coordinates. This setup represents a Hydrogen molecule (H₂).
• ecut 10.0: Sets the kinetic energy cutoff for the plane-wave basis set in Hartrees.
• nstep 10: Sets the maximum number of self-consistent field (SCF) cycles.
• toldfe 1.0d-6: The tolerance for the change in total energy, which determines when the SCF cycle has converged.

Dataset 2: Wavefunction Calculation

This section appears to be preparing for a subsequent calculation, possibly by generating a wavefunction (getwfk -1) on a slightly modified geometry.

ndtset 21
xcart:  -0.5   0.0 0.0
         0.5   0.0 0.0
xcart+  -0.025 0.0 0.0
         0.025 0.0 0.0
getwfk  -1
nband    1

Dataset 3: Geometry Optimisation

Here, the goal is to find the lowest-energy configuration for the atoms.

geoopt  "bfgs"
ntime   10
tolmxf  5.0d-4
xcart  -0.7   0.0 0.0
        0.7   0.0 0.0

• geoopt "bfgs": Specifies the algorithm for geometry optimisation (Broyden–Fletcher–Goldfarb–Shanno).
• ntime 10: The maximum number of optimisation steps.
• tolmxf 5.0d-4: The tolerance for the maximum force on any atom, which determines when the geometry is considered relaxed.

Dataset 4: Printing Electron Density

This dataset is very simple and instructs the program to output the electron density.

prtden 1

• prtden 1: A flag to enable the printing of the electron density to a file for later visualisation.

Dataset 5: Spin-Polarised Calculation

This dataset configures a spin-polarised calculation, which is necessary for systems with unpaired electrons.

nsppol 2
occopt 0
nband  1 1
occ    1.0  0.0
spinat 0.0 0.0 1.0

• nsppol 2: Enables a spin-polarised (collinear) calculation.
• occ / spinat: These parameters define the initial occupations and spin magnetisation on the atoms.

Dataset 6: Single Atom Calculation

Finally, this last dataset describes a simple calculation for a single, isolated Hydrogen atom.

natom 1
typat 1
xcart
   0.0 0.0 0.0

This input file provides a practical example of how different physical simulations can be chained together in ABINIT to investigate the properties of a simple system like Hydrogen.